As an alternative for the well known halogen based—flame retardant containing systems, in the last years the market is increasingly interested in halogen free flame retardant additives to be used in the production of corresponding flame-retardant compositions, generally speaking thermoplastic compositions.
Significant requirements for these products are: high flame retardant effectiveness in reinforced and unreinforced articles, pale intrinsic colour, good heat stability and good mechanical and electrical properties.
The known halogen-free flame retardant additives mostly used in thermoplastic polymers are:                Inorganic flame retardants belonging to the metal hydroxide group (Magnesium hydroxide and Aluminium hydroxide). These products must be used in large amounts to be effective and therefore show, as a drawback, that the mechanical properties of the related thermoplastic articles are dramatically damaged.        Melamine derivatives, like melamine cyanurate, melamine (poly)phosphate or melamine pyrophosphate. These products do not have sufficient thermal stability for overcoming the processing conditions of some thermoplastic polymer like for example polycarbonates, and therefore cannot be used as universal additives.        Organic derivatives of phosphoric acid such as phosphoric esters (valence state of P=+5). These products are not very effective, tend to bleed out on exposure to heat and are generally in a liquid status, being therefore difficult to handle. In addition they are not hydrolytically stable, therefore when used in Polycarbonates they produce high phenol values during processing and affect mechanical and thermal properties of final articles. Although many improvements have been introduced with the new high molecular weight products such as the condensate phosphate esters having a structure which is derived from Bisphenol A (JP open publication 6-228426), however the related polymer articles do not have fully satisfactory performances due to the unbalanced combination of flammability with impact resistance, heat stability and weather resistance.        Red-Phosphorus has been proven to be an effective flame retardant additive for opaque polycarbonate and/or PC/ABS alloys (U.S. Pat. Nos. 4,014,849; 6,448,316; 6,465,555; EP 0893475) but unfortunately its inherent deep-red colour makes the related polymer product difficult to penetrate the market.        Potassium salts of organic sulphonates, such as for example: potassium perfluorobutansulfonate, are also effective flame retardant additives for polycarbonate articles with a thickness of for example about 3 mm but they generally fail when used in the preparation of thinner articles, in filled polymers and in PC/ABS alloys.        Hypophosphorous acid metal salts as halogen free flame retardant agent, have been described since 1986 (U.S. Pat. No. 4,618,633) for fire proofing styrene containing polymers like: polystyrene (PS), high impact polystyrene (HIPS), styrene-acrylonitrile copolymers (SAN), acrylonitrile-butadiene-styrene polymers (ABS). These hypophosphorous acid metal salts, with particular regard to Calcium hypophosphite, have been described to be effective in polyester resin compositions when used in combination with nitrogen containing compounds like: melamine cyanurate, melamine polyphosphate, melamine borate, melamine pyrophosphate, ureaphosphate or similar products (U.S. Pat. No. 6,503,969; WO 09817720; DE 19904814; DE 10137930; EP 0919591).        